This invention relates generally to apparatus for and a method of exposing lithographic plates and, more particularly, to such apparatus and method which can be used to expose photosensitive or thermosensitive lithographic plates without the use of film.
Conventionally, photosensitive lithographic plates have been exposed to light through film. One type of apparatus designed to accomplish this in an automated manner is described in co-assigned U.S. Pat. No. 5,369,468, which is incorporated herein by reference. This apparatus is used particularly for newspaper printing operations to expose plates to light through photographic negatives of the newspaper pages. More recently, apparatus has been developed which uses laser technology to expose plates. This apparatus generally falls into three different categories; "flat bed" involving laser exposure of a plate on a flat bed; "internal drum" involving laser exposure of a plate mounted on an internal surface of a cylindric drum; and "external drum" involving laser exposure of a plate mounted on the external surface of a cylindric drum. Such apparatus are described in U.S. Pat. Nos. 5,174,205, 5,351,617 and 5,550,574, for example. By using laser technology to form an image directly on a plate, the need for film is completely eliminated. As a result, laser exposure systems are sometimes referred to in the trade as computer-to-plate imaging systems, or CTP systems. The present invention relates to such a system. In addition, the invention relates CTP systems which use lasers to form thermal images on thermosensitive plates.